Simon,
There is a reasonably standard approach to this - I have done these migrations a number of times (mainly to perform IP address migration when companies merge or need to move away from registered address space). OK, the problem is how do you introduce a new DHCP scope served by a new server, but that doesn't overlap with the existing scope? Depending on the ratio of number of simultaneous leases to the amount of address space on your subnet you might be able to reduce the size of the existing scope. If the scope on the old server can be resized to less than half of the subnet, you can then introduce the new server with the new non-overlapping scope. You would then just turn off the old DHCP server scope so that any clients that need to renew will be served by the new server when they next renew. (This approach is also used as a simple way to provide DHCP server redundancy - just have 2 non-overlapping scopes on 2 servers). If you don't have sufficient address space to do this then there are two things you can do 1.Reduce the lease time on the old server to say 2 hours. Next time clients renew then they will then be on a cycle to renew every 1 hour. Then, say overnight, remove the old DHCP server and introduce the new with the same scope range. Yes, there will be potential conflicts, but only for an hour (as all the clients will renew in that time). Most clients (well windows ones do) ping or arp for the address they are offered anyway and hence reject ones they see are being used. If you aren't a 24x7 operation this can work well as the machines will sort themselves overnight (or when they boot in the morning). 2. If you absoultely can't have any conflicts and the existing address range is constrictive you may be able to temporaily introduce a new subnet as a secondary address range on the same LAN. This would mean your router (hoping you have a local LAN router capable of reasonable performance) would then perform any routing necessary between the old and new subnets. The new DHCP scope would be based on this new secondary address range. Once you have this in place you could turn off the old DHCP server. Clients on the old and new subnets would still interwork (though via the router). When the clients on the old lease renew, they will then move to the new subnet. Once all the old leases have expired you could then optionally reintroduce a scope on the new DHCP server for the old subnet and basically migrate the clients back in the same fashion. You would turn off the new temporary scope and once all the clients have moved back to the original subnet you could remove the extra secondary address on your router. Martin Visser, CISSP Network and Security Consultant Consulting & Integration Technology Solutions Group - HP Services 410 Concord Road Rhodes NSW 2138 Australia Mobile: +61-411-254-513 Fax: +61-2-9022-1800 E-mail: martin.visserAThp.com This email (including any attachments) is intended only for the use of the individual or entity named above and may contain information that is confidential, proprietary or privileged. If you are not the intended recipient, please notify HP immediately by return email and then delete the email, destroy any printed copy and do not disclose or use the information in it. -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Michael Fox Sent: Thursday, 21 July 2005 8:47 AM To: [EMAIL PROTECTED] Cc: SLug Users Subject: Re: [SLUG] Changing DHCP servers On 7/21/05, Simon <[EMAIL PROTECTED]> wrote: > Hi all, > > I want to change from one DHCP server to another. The current one does > not give me enough control and is integrated into an e-smith server > (argghhhh, that was a bad idea! - another story). I can easily set one > up on one of my Linux servers, but how do I avoid IP conflicts as the > new server won't know about existing leases, or will those lease be > re-negotiated by the new server automatically? I don't know what > length the leases are at the moment, that is one of the issues. You should only run one DHCP server on your network/segment/subnet at a time. You would have more then 1 servicing the same ip range. I'd be inclined to setup the new DHCP to take over the role of the old one and then turn the old one completely off, then the new one on. And most machines should attempt to grab the same ip if available, and if not the new DHCP server will issue them new ones based on its leases available. -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
